A. Field of the Invention
This invention relates to the field of art of dielectric compensated level indicator control systems using capacitance measurement and reference probes for indication and control of substance level in a tank.
B. Background Art
It is well known that the level of a substance, i.e. a fluid or granular solid, in an open or closed tank or vessel can be measured and controlled by many fundamentally similar methods. Measurement and control is usually based on the concept that the change in fluid level in the tank is equivalent to displacing the top surface of the fluid.
In an earlier method of measurement and control, floats were used to detect and regulate the fluid level in a container. The method employs direct-actuated types of liquid level detectors and is applicable to open tanks or vessels which are subject to atmospheric pressure. However, when using closed tanks, water level is detected in a system under pressure. An arrangement used for this purpose includes one valve position at the lowest fluid level in the tank. Periodic opening of these valves will establish the presence of either steam or water at each valve permitting an inference to be drawn concerning the actual water level in the tank.
A prior computer-based control system used well known signal aquisition input instrumentation to obtain analog signals from sensors and transducers, such as capacitance probes, in the tank and transmits them to the computer. To close the fluid level control loop, D/A converters may be used to transmit the signals used to drive on/off fluid level controllers and actuators. Devices such as relays or stepper motors for opening or closing pneumatic fluid valves are also provided control signals from the computer along digital output channels for controlling the fluid flow into and out of the tank. The processor may, for example, compare the input signals from the fluid level transducers with upper and lower set point limits in order to control, in on/off, proportional, integral or differential modes, the fluid flow to the tank to maintain the desired liquid level within a predetermined range. Alarm monitoring and faulty transducer detection can also be performed by the computer.
Analog controllers may be used without a computer processor for controlling the level of fluid in the tank. The analog controller may either use its own set point reference voltage to control fluid input to the tank or it may accept fluid level set point limits from a central processor for the same purpose. Output devices such as strip chart recorders using properly scaled paper, calibrated meters with d'Arsonval movement and digital displays have all been used to show the amount and height of fluid in tanks.
Capacitance probes are commonly used to detect fluid levels in tanks. Systems employing these probes operate on the principle that the capacitance between two plates varies as the dielectric constant of the substance between the plates varies. Air has a dielectric constant of one and the substances measured by these systems have dielectric constants greater than one. If a probe is extended into a tank, the probe will serve as one plate of the capacitor and the tank will serve as the other plate. As various levels of substance are stored in the tank, varying regions between the probe and the tank will be filled with the substance and varying regions will be filled with air. This will cause the capacitance between the probe and the tank to vary as a function of the level of substance. Capacitance readings of the probe can therefore be scaled to indicate substance level.
Once such a system has been calibrated, the readings obtained are accurate only as long as the dielectric constant of the substance does not vary. If a different substance is stored in the tank or if temperature or humidity alter the dielectric constant of the substance in the tank, the system must be recalibrated. In order to solve this problem, a second probe has been placed in the tank in a position in which it will always be completely submerged by the substance. The capacitance reading of this probe therefore has been a function only of the dielectric constant of the material. However this method had required two transmitters, one to transmit the capacitance information from each of the probes, and two transmission channels, one for each transmitter.
When the capacitance information from the two probes has been processed by the control system an appropriate zero capacitance must be subtracted from each probe capacitance. The results for each capacitance must then be separately scaled. This has been done to make certain that the entire span of the output of the probes has been utilized in order to obtain accurate results. These two signals have been then divided into each other in a manner which caused the effects of the dielectric constant of the material to cancel out, causing the resulting ratio to be a measure of the level of substance which is independent of the dielectric constant of the substance.
Furthermore, the probes themselves must be recalibrated when substances with widely varying dielectric constants have been stored in the same container. The probes normally have an analog output span of 4 to 20 milliamps. A probe may therefore be calibrated to emit 4 milliamps when the container is empty and 20 milliamps when the container is full for some dielectric K.sub.1. If a substance with a dielectric constant of twice K.sub.1 is subsequently placed within the vessel the capacitance which resulted in 20 milliamps output would be achieved when the tank was 50% full. Recalibration of the control means can interpret this capacitance as 50% but measurements between 50% and 100% of the tank capacity would be beyond the span of the probe. To remedy this it has been necessary to adjust potentiometers within the probe head. The probe heads are located at the vessel, and adjusting the potentiometer often involves climbing the tank. The probe span recalibration is time consuming and inconvenient. Similarly, if a substance with a dielectric constant of one-half K.sub.1 is subsequently placed in the vessel the full range of the tank, 0% to 100%, would only occupy one-half the span of the probe output, 4 to 12 milliamps. Recalibration of the probe span at the probe head would be required to achieve the full range and accuracy of the probe.